When a vibrating body is mounted on a support body, it is often desired to minimise or eliminate the transmission of vibration from the vibrating body to the support body. It is known to reduce the transmission of vibration passively by providing vibration isolators of various forms between the two bodies. It is also known to reduce vibration actively, often in conjunction with the use of passive isolators. In the active control of vibration, sensors are attached to a body to measure its movements and these measurements are used to generate control signals for one or more devices for applying cancelling forces or movements to the body. It is known to provide this active control system on the vibrating body itself or on the supporting body.
In theory, the active control of vibration can be absolutely effective and reduce all vibrations to zero. However, this would require the provision of exactly the correct cancelling forces at exactly the correct timing. In practice, this is difficult if not impossible to achieve. Normally, both the vibrating body and the support will have a complex distribution of mass and rigidity and the vibrations to be cancelled will also be relatively complex. Accordingly, the calculation of the correct pattern of forces to be applied is complicated. Additionally, the calculation takes a certain amount of time. Accordingly, the cancelling forces actually applied will be only an approximation to the exact cancelling forces required and the timing of application of the forces may not be correct. Any attempt to increase the accuracy of the calculation will increase the time taken and any attempt to decrease the time taken will decrease the accuracy.
The problems are compounded in the common situation in which a vibrating body is mounted at several separate points on a supporting body. Active cancellation must be applied at each mount, thus each mount will tend to have several vibration sensors and several actuators to apply cancelling forces in different directions. If there are N mounts, each having M actuators and K sensors, then the control system must attempt to minimise the vibrations detected at KN sensors by the operation of MN actuators. However, since all the sensors and all the actuators are mounted on the same body (the vibrating body or the support body), the forces applied by each actuator will have an effect on the movements detected by each sensor, through the common body. Thus the control system must operate on an MN.times.KN matrix of interactions between actuators and sensors. Following calculation of this matrix, the control system must calculate MN control signals for the forces to be applied by each individual actuator in order to cancel the vibrations.
In the passive control of vibration, isolating arrangements are well known which include an intermediate body between the vibrating body and the supporting body. Such isolating arrangements comprise compound mounts designed so that the secondary mounting resonance is below the frequency at which vibration isolation is required. This means that, for effective vibration isolation at low frequencies, the intermediate body must be of relatively large mass.